The interactions of rat IgE with IgE receptors of various species will be studied with regard to specificity. Rat IgE receptor will be characterized with regard to its heterogeneity, its carbohydrate portion, its fragments, and its functional properties. The 30,000 MW macromolecule which appears to be associated with IgE receptor will also be characterized. Fc gamma receptors from rabbit and mouse macrophages as well as from human lymphocytes and neutrophils will be purified and characterized. Fc epsilon receptors and various Fc gamma receptors will be compared with respect to their structures, properties, and functions.